It is stated in the specification of British Pat. No. 796,862 that the molecular weight of formaldehyde polymer depends upon the amounts of traces of water, methanol, and formic acid present in the polymerization system. The specification of U.S. Pat. No. 3,017,389 also describes a process for polymerizing formaldehyde in the presence of an anionic initiator and of a chain transfer agent such as a monoester, monohydric alcohol, acid anhydride, amide, or imide. Among the chain transfer agents referred to in these patent specifications, while compounds having a so-called active hydrogen give adequate rates of chain transfer, compounds including monoesters, acid anhydrides, amides, and imides, which have no active hydrogen, possess the disadvantage of showing low rates of chain transfer, which results in unsatisfactory regulation of the molecular weight. Moreover, the use of any of these chain transfer agents including monohydric alcohols as well gives a polyoxymethylene of inadequate melt fluidity and processability. Thus, there is still great room for improvement in these techniques.
On the other hand, U.S. Pat. No. 3,436,375 discloses the polymerization of trioxane or formaldehyde using a cationic polymerization catalyst in the presence of a poly-hydric compound. In the polymerization by use of a cationic polymerization catalyst, there occurs a main chain scission reaction referred to as hydride shift reaction, causing a decrease in molecular weight of the polymer. Additionally, this type of main chain scission occurs significantly in the polymerization of formaldehyde, and it is therefore extremely difficult with a cationic polymerization catalyst to obtain a polyoxymethylene with a sufficiently high molecular weight.
Polyoxymethylenes are widely used as engineering thermoplastics and are usually shaped by using molding techniques such as injection and extrusion. Improvements of polyoxymethylenes in melt fluidity and in processability are expected to develop applications thereof to a great extent.